File photo of Thailand's coup leader General Prayuth Chan-ocha.

Nobody in the rubber-stamp legislature opposed the selection of the 60-year-old army chief, who ousted an elected government in a bloodless takeover on May 22.

The move by the general to shed his uniform and take the premiership is seen as cementing the military's control of the politically turbulent nation.

The junta, formally known as the National Council for Peace and Order (NCPO), has ruled out holding new elections until around October next year.

"The generals clearly do not plan to restore democracy," said Sunai Phasuk, senior researcher at New York-based Human Rights Watch.

"Instead of paving the way for a return to democratic civilian rule, the NCPO has granted itself unchecked authority to do almost anything it wants, including committing rights abuses with impunity."

Gen Prayuth, who is due to retire as army chief next month, is seen as a staunch opponent of fugitive former premier Thaksin Shinawatra, whose overthrow in an earlier coup in 2006 triggered Thailand's long-running political crisis.

Thaksin - whose sister, Yingluck, was dismissed as premier in a controversial court ruling just before this year's coup - fled Thailand in 2008 to avoid prison for a corruption conviction.

Gen Prayuth's appointment must be approved by King Bhumibol Adulyadej, although the palace's endorsement is seen as a formality as the former is seen as a fervent royalist.

Gen Prayuth is often described as the architect of an army crackdown on a pro-Thaksin "Red Shirt" rally in Bangkok in 2010 that left dozens dead.

Before seizing power, the golf lover and father of twin daughters had said he would not allow Thailand to become another "Ukraine or Egypt".

The United Nations' human rights office on Wednesday warned of "chilling effects" on freedom of expression under the junta, following recent arrests and jail sentences for insulting the monarchy.

Critics say the royal-slur legislation has been politicised, noting that many of those charged in recent years were linked to the "Red Shirts", who are broadly supportive of Thaksin.